The invention relates to a device for forming a quasi-neutral beam of oppositely charged particles.
Such devices are notably used for plasma thrusters (application to satellites for correcting a trajectory, space probes, . . . ), devices for depositing particles on a target (vapor deposition, for example; field of microelectronics), devices for etching the target, devices for treating polymers or further devices for activating the target surface.
Typically, such a device comprises a chamber, means for introducing an ionizable gas into the chamber, means for ionizing the gas in order to form the plasma and means for extracting and accelerating charged particles from the plasma out of the chamber.
In the field of electric propulsion, there exist different techniques for ensuring the acceleration of the craft provided with the plasma beam generator, which is then assimilated to a plasma thruster.
Thus, for a plasma thruster, the plasma of which is formed with positive ions and electrons, it is possible to extract and only accelerate the positive ions out of the chamber and to ensure electro-neutrality of the beam of positive ions after leaving the chamber by injecting electrons downstream from the outlet of the chamber.
Ensuring the electro-neutrality of the beam at the outlet of the chamber is actually indispensable for avoiding that spacecraft be electrically charged, the current of the ion beam notably not being limited by the space charge.
This type of plasma thruster however has the drawback of applying an ancillary source of electrons for ensuring this electro-neutrality, an ancillary source which is generally the cause of a lack of reliability.
In order to ensure this electro-neutrality by increasing the reliability (therefore doing without the ancillary source of electrons), several routes have been contemplated.
A first route is to produce a plasma including positive ions, negative ions and electrons and to filter out the electrons, within the chamber, in order to only obtain at the outlet of the chamber or quasi only positive and negative ions. The oppositely charged particles of the beam are thus formed with positive ions and negative ions.
A second route is to produce a plasma including positive ions and electrons and to provide means for extracting and accelerating the positive ions and the electrons at the outlet of the chamber in order to ensure this electro-neutrality. The charged particles of the beam are thus formed with positive ions and electrons.
Solutions corresponding to the first described route above are proposed in documents WO 2007/065915, WO 2010/060887 or further WO 2012/042143.
All these solutions should apply an electronegative ionizable gas able to generate positive, negative ions and electrons as well as a means for filtering out the electrons in order to obtain at the outlet of the chamber only or practically only positive and negative ions.
In document WO 2007/065915, two grids 3, 4 in contact with the plasma are used as an extraction and acceleration means, which are located at the outlet of the chamber in a same plane (one at the top, the other one at the bottom) including one which is negatively biased and the other one is positively biased.
FIG. 1 is a representative diagram of the device proposed in WO 2007/065915. In this figure, the chamber 1 comprises a plasma with positive ions A+, negative ions A−, and electrons e−. The means for filtering the electrons bears the reference 2.
Extraction and simultaneous acceleration of the positive and negative ions is thereby obtained, ensuring electro-neutrality of the ion beam after the outlet of the chamber.
However, this solution is delicate to apply because of the presence of grids having opposite biases. Indeed, the presence of these grids with opposite biases may imply significant curvatures of the beams stemming from each grid.
Document WO 2010/060887 proposes an improved solution as compared with the one of document WO 2007/065915, for which two different gases are provided instead of one in WO 2007/065915. One of these gases is electronegative and the other one may either be electropositive or electronegative.
In document WO 2012/042143, the application of an extraction and acceleration grid 5 is proposed, powered by an alternatively positive and negative voltage source, via the voltage source 6. With this grid 5 is associated another grid 7 which is connected to the ground 8.
When a positive potential is applied on the grid 5, the plasma potential becomes positive and the positive ions A+ are accordingly accelerated towards the other grid 7 which is connected to the ground. Indeed, under these conditions, a positive sheath is formed at the grinds 5, 7, which allows acceleration of the positive ions. The sheath is a space which is formed between each grid 5, 7 and the plasma wherein the density of the positive ions differs from the density of the negative ions. Under these conditions, the extraction and acceleration of the negative ions is blocked.
Next, when a negative potential is applied onto the grid 5, the plasma potential becomes negative and the negative ions A− are accelerated towards the other grid 7. More specifically, after having applied a positive potential on the grid 5, the positive sheath disappears rapidly (about 1 microsecond) and a negative sheath is formed under the effect of the negative bias of this grid 5. Under these conditions, the extraction and the acceleration of the positive ions is blocked.
Depending on the bias of the grid 5, it is thus possible to accelerate and extract either the positive ions or the negative ions.
A representative diagram of the device proposed in document WO 2012/042143 is illustrated in FIG. 2 (a). The electronegative gas is noted as A2 and the means for filtering the electrons 2. RF′ designates here the means which allows generation of the plasma from the electronegative gas A2 injected into the chamber 1. This is a source of an alternating sign-wave magnetic field emitting in the field of radiofrequencies.
Here, one does not have the drawback of having two oppositely biased grids, like in documents WO 2007/065915 and WO 2010/060887. However the positive ions A+ and negative ions A− being extracted successively, optimization of the shape of the voltage signal generated by the alternating voltage source 6 connected to the grid 5 is proposed for ensuring at best the electro-neutrality of the ion beam at the outlet of the chamber. This alternating voltage source 6 may make use of the measurements of a probe S in the output beam and/or the RF′ signal used for generating the plasma.
This optimized signal is illustrated in FIG. 2(b).
With this optimized signal, good electro-neutrality of the beam is obtained at the outlet of the chamber 1, but only on average.
Indeed, the fact of successively extracting the positive ions, and then the negative ions and vice versa does not always give the possibility of obtaining a constantly neutral beam. Accordingly, the potential of the thruster varies overtime, depending on the shape of the signal illustrated in FIG. 2(b).
Moreover, it should be noted that all the ion-ion extraction devices, the use of an electronegative gas, which is generally highly reactive (presence of fluorine, chlorine, . . . ) limits the lifetime of the device.
Further, the solutions proposed in documents WO 2007/065915 (FIG. 1), WO 2010/060887 and in FIGS. 2(a) and 2(b) (WO 2012/042143) are limited to the ion-ion extraction, but cannot be contemplated for extracting ions-electrons.
Another solution corresponding to the second route described above is proposed in the article of S. V. Dudin & D. V. Rafalskyki, “On the simultaneous extraction of positive ions and electrons from single-grid ICP source”, A letters Journal Exploring the frontiers of Physics, EPL, 88 (2009) 55002, p 1-p 4.
This solution consists of applying an electrode 9 in the core of the chamber 1 (therefore within the plasma), the electrode 9 being powered by a radiofrequency voltage source 10 (RF; source of an alternating sign-wave voltage at a frequency comprised in the range of radiofrequencies) via a capacitor 11 and of associating with it a grid 7″, located at the outlet of the chamber 1, in contact with the plasma and connected to the ground 8.
Reference may be made to FIG. 3, which is a representative diagram of the device. In this FIG. 3, RF′ represents a radiofrequency source (for example one or several coils) for ionizing the gas and thereby forming a plasma including positive ions and electrons. The means 12 is a chamber in vacuo in which are installed means giving the possibility of characterizing the ion beam stemming from the chamber 1, which are not involved in the extraction and acceleration of the ions.
The operation of the device is the following.
By design, the electrode 9 has a clearly greater surface area than that of the grid 7″ located at the outlet of the chamber 1 and connected to the ground 8.
Generally, applying an RF voltage on an electrode having a larger surface area than the grid 7″ has the effect of generating at the interface between the electrode 9 and the plasma on the one hand, and at the interface between the grid 7″ and the plasma on the other hand, an additional potential difference, which adds to the RF potential difference. This total potential difference is distributed on a sheath. Here, the sheath is a space which is formed between the grid 7″ or the electrode 9 on the one hand and the plasma on the other hand wherein the density of positive ions is greater than the density of electrons. This sheath has a variable thickness because of the RF signal applied to the electrode.
In practice, the major portion of the effect of the application of an RF signal on the electrode 9 is however located in the sheath of the grid 7″ (the electrode-grid system may be considered as a capacitor with two asymmetrical walls, in this case the potential difference is applied on the portion having the lowest capacitance therefore the smallest surface).
In the presence of the capacitor 11 in series with the RF source, 10, the application of the RF signal has the effect of converting the RF voltage into a constant DC voltage because of the charge of the capacitor 11, mainly at the sheath of the grid 7″.
This constant DC voltage in the sheath of the grid 7″ implies that the positive ions are constantly accelerated. Indeed, this DC potential difference has the effect of making the plasma potential positive. Accordingly, the positive ions of the plasma are constantly accelerated towards the grid 7″ (at the ground) and extracted from the chamber 1 with this grid 7″. The energy of the positive ions correspond to this DC potential difference (average energy).
The variation of the RF voltage, 10, gives the possibility of varying the RF+DC potential difference between the plasma and the grid 7″. At the sheath of the grid 7″, this is expressed by a time-dependent change in the thickness of this sheath. When this thickness becomes smaller than a critical value, which happens during a lapse of time with regular intervals given by the frequency of the RF signal, the potential difference between the grid and the plasma approaches the value zero (therefore the plasma potential approaches the value zero, the grid being at the ground), which gives the possibility of extracting electrons.
In practice, the plasma potential below which the electrons may be accelerated and extracted (=critical potential) is given by Child's law, which connects this critical potential to the critical thickness of the sheath below which this sheath disappears (sheath collapse).
As long as the plasma potential is less than the critical potential, then there is acceleration and simultaneous extraction of the electrons and of the ions.
Although the extraction of electrons is only conceivable over a certain lapse of time during a period of the RF signal applied to the electrode 9, this article shows the possibility of complete compensation for the positive charge of the ions and therefore good electro-neutrality of the beam at the outlet of the plasma chamber.
Moreover, quasi-simultaneous acceleration and extraction of the positive ions and of the electrons are obtained during a period of the RF signal, unlike the solution proposed in WO 2012/042143, whether for the ion-ion extraction or the ion-electron extraction.
The technique proposed in this article is therefore very different from the ones which are proposed in documents WO 2007/065915, WO 2010/060887, and WO 2012/042143 (notably that of FIG. 3, for the ion—electron extractions), by applying a single grid (grounded) in contact with the plasma and a capacitor 11, which provides a continuous component to the potential difference in the sheath, in series with an RF voltage source 10.
A drawback of this technique is that there is a lot of losses of accelerated positive ions, i.e. positive ions accelerated to a high energy, but which do not pass through the orifices of the grid. This causes more rapid wear of the grid and accordingly limits the lifetime of this grid. In the case of an application to a plasma thruster (satellite, space probe, . . . ), this drawback may become critical. In practice, in order to limit this drawback, ions therefore should be applied, for which the energy is less than 300 eV.
Moreover, this technique may not operate for the ion-ion extraction and acceleration.